The present disclosure relates to a display device emitting light using a phenomenon of organic EL (Electro Luminescence) and being transmissive, and a display and an electronic unit provided with the display device.
An organic electroluminescent display device has been receiving attention as a promising candidate for a next-generation display device, and is recently receiving attention also as a possible application for a transparent display.
The transparent display is a promising device of offering a new world with a sense of unity between Augmented Reality (AR), in other words, the in-screen world, and another world visible through the screen. Such a transparent display has been under active technology development (for example, see Non-Patent Literature 1 (Young Woo Song, Sansung SDI, Yongin, Korea, SID 10 DIGEST, 144-147), Non-Patent Literature 2 (Jinkoo Chung, Samsung Mobile Display, Yongin, Korea, SID 10 DIGEST, 148-151), and Non-Patent Literature 3 (Sang-Hee Park, ETRI, Daejeon, Korea, SID 10 DIGEST, 245-248)). As an exemplary AR technology, Non-Patent Literature 1 describes the approach of improving the transmittance of a panel itself, for example. To be specific, the transmittance of the entire panel is increased in the previous approach by, in a pixel 100 including light-emitting regions 101 and drive circuits 102 over the entire surface thereof as shown in FIG. 14A, reducing the size of the light-emitting regions 101 to provide an empty transparent region 103 as shown in FIG. 14B, for example.
The concern about the transparent display is to ensure the viewability thereof (for example, to increase the contrast thereof) as being transparent. To get around the disadvantage as such, a popular approach is to concentrate luminous fluxes toward the front while ensuring a certain level of light transmission property for display by utilizing the strong cavity effect using a semi-transmissive metal film, for example. However, with the resulting transparent display manufactured by the method as above, light of the display section hardly reaches the rear surface side of the panel. Therefore, the side in one direction (the side of the front surface) is indeed transmissive, but the rear surface side is like a mirror with the extremely poor viewability including display. Such a transparent display is thus hardly transparent.
Moreover, with the transparent display structured as described above, since the viewability is ensured by using the strong cavity effect, the viewing angle dependence of chromaticity and that of luminance are both high even on the front surface side with the good viewability. This thus results in a disadvantage of failing to fully utilize the excellent points of the transparent display that is supposed to allow viewing of the same display screen no matter from which direction.
To get around such a disadvantage, in Japanese Unexamined Patent Application Publication No. 2006-165034, for example, proposed is an organic EL device improved in light extraction efficiency with a reduced color tone difference of emitted light between a pair of transparent electrode sides. This proposal is made by defining the length of cavity between the transparent electrodes on both surfaces and the light-emitting position. This organic EL device allows light extraction from both sides of first and second transparent buffer regions, which are provided outside of the transparent electrodes, respectively. Between these transparent electrodes, provided is an organic layer including a light-emitting layer. To be specific, in the organic EL device, the refractive index step is set to be about 0.6 or smaller in any boundary surfaces existing at positions where the distance from the light-emitting interface of the light-emitting layer is smaller than the coherence length Lc, which is calculated by PL (PhotoLuminescence) spectrum of light coming from the light-emitting interface.
In Japanese Unexamined Patent Application Publication No. 2005-310469, proposed is the approach to improve the efficiency of light emission through improvement of adhesion by processing the interface between a transparent electrode formed on the surface opposite to the substrate and an organic layer using a silane coupling agent.